1. Field of the Invention
This invention relates to wireless communications.
2. Description of Related Art
Position determination has become significantly easier and more accurate since the development of positioning satellite systems. One example of a system of positioning satellites is the NAVSTAR Global Positioning System (GPS) (as described in Global Positioning System Standard Positioning Service Signal Specification, 2nd edition, Jun. 2, 1995, United States Coast Guard Navigation Center, Alexandria, Va.). Another example of an existing system is the GLONASS GPS maintained by the Russian Republic. Positioning satellite systems under planning include the European GALILEO proposal. GPS receivers are currently available for use in aircraft, ships, and ground vehicles and for hand carrying by individuals.
The NAVSTAR GPS provides for thirty-two satellites or xe2x80x98space vehiclesxe2x80x99 (SVs) (twenty-four are currently active) that orbit the earth in six orbital planes (four satellites in each plane). The SV orbits repeat almost the same ground track as the earth turns beneath them each day. The orbital planes are equally spaced and inclined with respect to the equatorial plane, thus ensuring that a line-of-sight path exists to at least five SVs from any (unobstructed) point on the earth.
Each SV carries a highly accurate atomic clock that is synchronized to a common time base. Ground-based monitor stations measure signals from the SVs and incorporate these measurements into orbital models for each satellite. Navigation data and SV clock corrections are computed for each satellite from these models and are uploaded to each SV. The SV then transmits a navigation message that includes information relating to its position.
Each SV transmits its navigation message at a data rate of 50 bits per second via a direct sequence spread spectrum signal (DSSS) that is BPSK (binary phase-shift-keying) modulated onto a 1.57542-GHz carrier (also called L1 frequency). To spread the signal, each SV uses a different one of thirty-two pseudo-random noise (PRN) sequences (also called coarse acquisition or C/A codes) that have a chip rate of 1.023 MHz and a length of 1023 chips. The spreading codes are aligned with the common time base and repeat every millisecond.
A GPS receiver calculates its position by combining data from the navigation message (which data indicates the position of the SV) with the delay of the signal received from the SV (which indicates the position of the receiver relative to the SV). Because of offsets in the receiver""s time base relative to the GPS time base, signals from at least four SVs are typically required to resolve a position in three dimensions, although signals from additional SVs (if available) may be used to provide better accuracy.
Problems in GPS signal detection may occur when a GPS receiver cannot receive a line-of-sight signal from a sufficient number of SVs. In obstructed environments (e.g. indoors or underground), therefore, it may be difficult or impossible for a GPS receiver to make an accurate position determination.
A pseudolite is a terrestrial transmitter that receives one or more GPS signals and generates and transmits a C/A waveform at the GPS L1 carrier frequency. In the NAVSTAR GPS system, PRN sequences 33 through 37 are not assigned to satellites and may be used by a pseudolite to generate and transmit the C/A waveform. If the timing and position of a pseudolite are known with high precision, then the transmitted C/A waveform may be used to make a position determination.
Pseudolites may be used to augment GPS coverage. Unfortunately, pseudolites require a line-of-sight signal from one or more GPS satellites and are useful only where a GPS signal is available.
A method of wireless communications according to one embodiment of the invention includes obtaining a first time base from a signal received from a ground transmitter. For example, obtaining the first time base may include obtaining a code phase of the received signal and/or decoding a time information message from the received signal. Obtaining the first time base may also include synchronizing an oscillator or adjusting a counter or code generator. In one example, the first time base is obtained from a signal received from a base station of a network for cellular telephony (e.g. a CDMA base station).
Such a method also includes applying a predetermined offset to the first time base to obtain a second time base. The predetermined offset is based on a propagation delay of the received signal. The predetermined offset may also be based on signal processing delays and/or other signal transmission delays. Obtaining the second time base may include synchronizing an oscillator or adjusting a counter or code generator.
Such a method also includes generating a timing signal that has a code phase based on the second time base. For example, the code phase of the timing signal may be aligned to a time base of a positioning satellite system (e.g. the NAVSTAR GPS).